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"Chopsticks" the Spider Robot 



.1 



Make Projects 



build, hack, tweak, share, discover. 



Chopsticks 11 the Spider Robot 



Written By: OddBot 



TOOLS: 



#1 Phillips Screwdriver (1) 

Kettle (1) 

Side cutters (1) 

metal skewer or piece of thick wire (1) 



PARTS: 



Spider controller with HP servo shield 

111 

12Kg/cm servo (8) 

5Kg/cm servo (16) 

self tapping screws (200) 

self tapping screws (50) 

self tapping screws (20) 

Pan / Tilt kit (2) 

Polymorph (500g) 

disposable Chopsticks (20+) 

7.2V / 7.4V Rechargeable battery pack 

ill 

High current switch (1) 

Fuse or circuit breaker (1) 

Cable ties (4) 

Cable ties (24+) 



SUMMARY 



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"Chopsticks" the Spider Robot 



Multi-limbed robots such as hexapods can be very complex and expensive. This is a 
relatively cheap and simple 8-legged robot made with chopsticks, Polymorph and an 
Arduino-compatible controller. Most of the servos cost under $8 if you shop around. I have 
seen the 12Kg/cm servos for as little as $13 at HobbyKing. 

The construction is pretty easy but takes time as you often have to hold parts in position and 
wait for the Polymorph to cool. Patience is definitely a virtue. Cold weather will speed things 
up. 

This project will show you how to make an 8-legged spider robot. As you need 3 servos for 
each leg the servos are the most expensive component. You can make fewer legs for a 
quadruped or hexapod if you wish to save money. 

I used two pan/tilt kits fitted with IR compound eyes for close-range object tracking. These 
are relatively cheap and give your robot the ability to play with people which adds a lot to its 
personality. See the videos below. 



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"Chopsticks" the Spider Robot 



Step 1 — Prepare your workspace. 




• A glass-top table is best when using Polymorph. You need a flat workspace that the 
Polymorph won't stick to. If you have a wood table then keep the top wet or put a sheet of 
glass on top. 

• You will need a rag, a constant supply of hot water, a bowl for the hot water (not plastic as 
the Polymorph may bond to it) and a cup of cold water (ice will speed things up) for cooling 
the Polymorph. Coffee is optional. :) 

• When you dip hot Polymorph in cold water for a few seconds the outside will cool rapidly 
forming a milky, flexible skin. This will prevent the Polymorph from bonding to your servos. 
The exact amount of time required will depend on how hot your Polymorph is and how cold 
the water is. A slightly milky appearance is your best indicator. 

• For each leg segment I used a 230mm chopstick cut in half, each segment being about 

1 15mm in length. Cut 16 chopsticks in half with the side cutters now so they are on hand 
when the Polymorph is hot. 

• I have used pan-head self-tapping screws for this project. Polymorph is perfect for self- 
tapping screws and the pan heads eliminate the need for washers. 



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"Chopsticks" the Spider Robot 



Step 2 — Prepare your Polymorph. 




• Polymorph is actually Polycaprolactone, a non-toxic, re-usable, biodegradeable plastic. 
You can read about it here. 



A 



• Boiling water can be dangerous. This project should not be attempted by children 
without adult supervision. 

• Put in only as much Polymorph as you need. If you put in too much at once you can end 
up with a big lump that is slow to reheat. 

• When you put the Polymorph granules into the hot water they will quickly become 
transparent and sticky. Once all the granules are transparent use a chopstick (because 
the water is very hot) to remove some from the bowl. Squeeze it gently in your hands to 
remove water trapped inside. 

• The Polymorph is now a putty that can be easily shaped by hand. At this time it will also 
stick to objects so be careful where you put it. It can bond to the plastic servo housing if it 
is too hot. As it cools it will slowly harden and become white again. When completely 
cooled it is a plastic similar to nylon. 



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"Chopsticks" the Spider Robot 



Step 3 — Making a hip joint. 




• The first piece to make is a cradle for the thigh servo. This piece allows the thigh servo to 
be attached to the hip servo. As the robot can be quite heavy this part needs to be quite 
strong. The thigh servo clips into this piece tightly with 4 mounting screws ensuring the 
servo cannot unclip. 

• Roll up a ball of Polymorph about 30-35mm in diameter. Then roll the ball into a thick rod 
about 40mm long. Dip this in the cold water for a few seconds to ensure that the 
Polymorph does not bond to your servo. 

• Wrap this rod around your servo as shown in the photo making sure you have enough 
Polymorph around the servo mounting holes for the mounting screws. 

• Press your servo horn gently into the Polymorph making sure it is reasonably well 
centered. Do not press too hard as you need at least 3-4mm of Polymorph behind the 
servo horn for the mounting screws to go into. 

• As the Polymorph cools, remove the servo horn and poke a hole about 6mm in diameter in 
the center of the servo horn impression. This will allow you to insert the screw that holds 
the servo horn to the servo. 

• Once it has fully cooled you should be able to unclip the servo. If your Polymorph was too 
hot and bonded to the servo then use a flat-blade screwdriver or a knife and gently pry the 
Polymorph away from the servo. 

• Mount your servo horn with 2x6mm pan-head screws. Attach the servo cradle to the hip 
servo and align it so that when the hip servo is in the center position the thigh will be 
perpendicular to the body. 



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Page 5 of 21 



"Chopsticks" the Spider Robot 



Step 4 — Making a thigh - mount the knee servo. 




• The next piece is the thigh. The chopsticks are usually tapered with a thick end to be held 
and a thin end for holding the food. Use two thick sections for the thigh. We will use the 
thinner sections for the legs. My chopsticks are square at the thick end and round at the 
thin end but the shape is not important. 

• Start with a small ball of Polymorph about 10-1 5mm in diameter and roll this into a thick 
rod about 40mm long. Press your chopsticks into the ends of the Polymorph rod while it is 
very hot so they bond. Press the Polymorph fully around the ends of the chopsticks so it 
covers about 10mm of the chopsticks. 

• Dip the Polymorph into the cold water for a few seconds and then place a 5Kg/cm knee 
servo between the chopsticks. Shape your Polymorph so that the servo is held firmly 
between the chopsticks with a solid section for the servo mounting screws to dig into. 

• Hold everything in place for a few minutes while the Polymorph cools and hardens. Once it 
is completely cool the servo should be a tight fit between the chopsticks but easily 
removed. Insert two 8x2mm mounting screws to hold the servo in place. 

• To make the small servo mount for the other end, get a small ball of Polymorph about 10- 
12mm in diameter and roll it into a thick rod about 20mm long. Dip this in cold water for a 
few seconds. Now press it into the other servo mount and make sure it goes around the 
chopsticks enough so that when cool it can slide along the length of them without falling 
out. This makes it easy to remove the servo if you need to replace it. 

• Once the Polymorph has hardened use two 8x2mm pan-head screws to fix the servo to the 
new servo mount. 



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"Chopsticks" the Spider Robot 



Step 5 — Making a thigh - the thigh joint. 




• Now get a big ball of Polymorph about 25mm in diameter. Squash it flat and wrap it around 
the other end of the chopsticks while the Polymorph is very hot. You should have a flat 
area big enough to mount another servo horn. 

• Dip the Polymorph in cold water for a few seconds and then gently press a servo horn into 
the Polymorph so it sits flat and makes an impression in the Polymorph. While the 
Polymorph is soft, create a 6mm-diameter hole in the center for the servo horn mounting 
screw to go through. 

• Once the Polymorph has cooled mount the servo horn using 2x6 pan-head screws. Make 
sure the hole for the servo horn mounting screw lines up with the center of the servo horn. 

• Your thigh segment is now complete and can be mounted on the thigh servo. Align your 
thigh so that when the thigh servo is in center position the thigh is parallel to the ground. 



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"Chopsticks" the Spider Robot 



Step 6 — Making a leg. 



^ttsL ^k 


-J 





• The leg is made using two of the thinner halves of the chopstick. Get a ball of Polymorph 
about 25mm in diameter and poke your two chopsticks into it, one on either side, while it is 
very hot. Knead the Polymorph around the chopsticks and hold the other ends of the 
chopsticks together to form a "V" shape. 

• Wet a spot on the table so that the Polymorph wont stick and then squash your ball into a 
thick disc on the table. Poke a 6mm-diameter hole in the center of the disc for your servo 
horn mounting screw. 

• While the Polymorph is still soft, gently press a servo horn into the Polymorph to leave an 
impression. Make sure the hole in the center lines up. 

• While the top of the leg is cooling get a small ball of hot Polymorph and squash it around 
the other end of the chopsticks to make a foot. 

• Once the leg has cooled, mount the servo horn with 2x6mm pan-head screws. Mount the 
leg so that when the knee servo is in its center position the leg is at 90 degrees to the 
thigh. 

• Repeat steps 3-6 until you have as many legs as required. Remember to make left and 
right legs in equal numbers. 



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"Chopsticks" the Spider Robot 



Step 7 — Joining your legs into pairs. 




• Join your left and right legs together in pairs. Start by making another ball of Polymorph 
about 15-20mm in diameter. Roll it into a short rod about 40mm long. 

• While the Polymorph is very hot, press a thick section of chopstick into each end and 
knead the Polymorph so it covers about 10mm of each chopstick. 

• Dip the Polymorph in cold water for a few seconds and then place the chopsticks either 
side of a left hip servo. Note this time I've put the Polymorph on the other side of the servo 
mount. This allows my mounting screws to be inserted from the top of the robot instead of 
the bottom. 

• Make sure the chopsticks are pressed tight against the sides of the servo and there is 
plenty of Polymorph for the servo mounting screws to dig into. Also check your clearance 
so that the hip can swing freely. 

• Once the Polymorph has cooled, use 2x8mm pan-head screws to secure the hip servo. 
Repeat the process at the other end with the right hip servo. 

• There should be a gap between the left and right hip servos in the center of the frame you 
just made. Roll up a ball about 20mm in diameter and dip it in cold water for a few 
seconds. Press it into the center between the two servos. 

• While this center piece is cooling, make an impression in the center with your finger so the 
Polymorph is only about 4mm thick in the center. Use a skewer to poke a small hole about 
3mm in diameter through the center for a mounting screw to go through. 



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"Chopsticks" the Spider Robot 



Step 8 — Add a spine. 






• A spine can be made from two whole chopsticks. Roll up a hot ball of Polymorph about 20- 
25mm in diameter and press your two chopsticks into it. Knead your Polymorph around the 
sticks so they are parallel with a gap of about 3-4mm in the center. Dip it in cold water for 
a few seconds and check that it fits between the servo pairs. Adjust your center gap for a 
neat fit. 

• Do the same at the other end so that your chopsticks remain parallel. Wet the table top 
slightly so the Polymorph doesn't stick and press your spine gently on the table top to 
ensure it is completely flat when the Polymorph hardens. You may wish to extend the ends 
slightly so you have a section of Polymorph where you can mount sensors, switches etc. 

• Now that the spine is flat and rigid you can add two more smaller balls of Polymorph at 
equadistant points along the spine. 

• Make sure each ball is hot so it bonds with the chopsticks and knead it evenly around 
them. Dip it in cold water and then check that it fits between the left and right hip servos. 

• Once again wet the table top and press the spine gently against the table top so that the 
spine sits completely flat on the table while the center pieces cool. 

• Use 2.3x1 2mm screws to fix your leg pairs to your spine. 



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"Chopsticks" the Spider Robot 



Step 9 — Mounting the battery. 




• Mount your battery in the center of 
the spine using 4 large cable ties. 
Make sure your battery cable goes 
to the back of the robot as this is 
the most convienient place to 
mount your power switch. Try to 
center the weight of the battery so 
that all legs support the weight 
equally. 

• If your battery will not fit neatly 
between the servos then you may 
need to make an additional bracket 
with Polymorph and chopsticks to 
mount your battery on. 

• Remember that your battery may 
get hot when fast-charging. If it is 
sitting on Polymorph then it might 
melt the Polymorph. Place small 
sections of chopstick between your 
battery and the Polymorph for 
thermal insulation. 



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"Chopsticks" the Spider Robot 



Step 10 — Framework for the electronics. 




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"Chopsticks" the Spider Robot 



• Now you need to build a frame to mount your controller and other electronics on. Plan this 
stage carefully allowing for your choice of battery, charging circuitry, controller etc. If you 
do make a mistake the chopsticks are cheap. Just put the frame in hot water to reuse the 
Polymorph. 

• As the legs can fold up, your frame should hold your circuitry above the legs when they 
are folded. This makes the electronics easy to access at all times without limiting leg 
movement. 

• I've used two whole chopsticks for the long sections. The cross sections are 80mm, about 
10mm wider than my controller. The vertical pieces making the "V" shape are my standard 
chopstick halves (115mm). 

• Wet a section of your table so the Polymorph won't stick and make a rectangular frame 
using two full-length chopsticks and 3 shorter lengths (80mm in my example). Start with 
the 4 outer corners and then the middle section allowing for the size of your controller 
board. 

• In the section where your controller will mount, squeeze out the Polymorph to make some 
mounting tabs in the corners. You can see in the second photo how this helps mount the 
controller. I've used some of the mounting equipment that came with the servos as 
spacers. 

• Before the Polymorph hardens, set your controller in its place. You can use the controller 
to make sure the frame is square and use the skewer to make indentations where the 
mounting screws will go. Let the frame harden while sitting flat on the table. 

• Now add your "V" sections made from chopstick halves. Choose your placement carefully 
allowing for the position of your battery. With your frame on the table, hold your "V" 
sections in place until the Polymorph sets. 



© Make Projects www.makeprojects.com Page 13 of 21 



"Chopsticks" the Spider Robot 



Step 11 — Running the power cables. 




Battery Battery 
Positive Negative 




Robot Rob at 
Positive Negative 



73V 



5-;. to- 
Bh M 



Chopsticks Power Schematic 



A 



• Use more Polymorph to mount your frame onto the spine of the robot. You might choose to 
simply wrap Polymorph around the spine and tip of the "V" sections fusing it all into a 
single piece as I did or you might be clever and make it so your frame clips onto the spine 
and is then locked in place with some screws. 

• Now is a good time to run your power cables, mount your fuse, switch and any special 
recharging circuitry. As I am using a 4500mAH NiMH battery my recharge circuit is just a 
2.5mm socket with a 6.8 Ohm 3W resistor in series. This limits my charging current to 
about 700mA when I plug the robot into a 12V 1 A power supply. 

• If you use a LiPo battery then you must use the correct charger for that battery. 
LiPo batteries can catch fire or explode if not charged correctly. They cannot be 
"trickle charged". 

• Use good quality, heavy-duty cable (AWG 8 or 9) for your power cable. If your cable is too 
thin then it will cause your voltage to drop when your servos draw a lot of current which 
can cause your controller to reset. 

• I have used a solid-state high-powered switch from DAGU controlled by D12 of my 
controller. This allows my controller to power the servos after it initializes. The diagram 
shows the best way to connect your power. 

• I've made a small platform out of Polymorph at the back of my robot to mount my switch. I 
simply squashed some Polymorph between the flat table top and the bottom of a ceramic 
dinner plate to get a flat sheet about 3mm thick. I pressed my switch gently into the soft 
Polymorph to get an impression of the mounting holes. Use the skewer to poke holes for 
the screws. When the platform had hardened I cut it to shape using the side cutters. 

• By holding just the back of the platform in hot water I made it soft so it would bond easily 
with additional Polymorph I had wrapped around the "V" section of my frame. I then held it 
in place with a cable tie until it had fully hardened. 



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"Chopsticks" the Spider Robot 



Step 12 — Mounting the controller. 






• Mount your controller on the frame using 2x8mm screws. I used some of the servo 
mounting hardware as spacers to raise the board slightly. 

• The servos I used have a 6V rating so I have used diodes to drop 7.2V down to 6V. I have 
put 4x 3A diodes in parallel to make a 12A diode. I then put two of these diodes in series to 
give me a total voltage drop of 1.2V. I soldered my diodes together freeform and cable-tied 
them to the frame. This is not required for 7.2V servos. 

• You can make your own shield from standard prototype PCB if you wish. Unlike standard 
Arduino controllers the Spider uses standard pin spacing. 



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Page 15 of 21 



"Chopsticks" the Spider Robot 



Step 13 — Align your servos. 




• Connect your servos to your controller. Pay careful attention to the orientation of the plugs. 
In my sample code the legs are numbered the same way as a microchip. Front left is Ieg1 , 
rear left is Ieg4, rear right is Ieg5 and front right is Ieg8. 

• The digital pins for the servos are: D26 thigh4, D27 thigh3, D28 thigh2, D29 thighl D30 
knee 4, D31 knee 3, D32 knee 2, D33 knee 1 D34 hip 4, D35 hip 3, D36 hip 2, D37 hip 1 
D38 hip 8, D39 hip 7, D40 hip 6, D41 hip 5 D42 knee 8, D43 knee 7, D44 knee 6, D45 knee 
5 D46 thigh8, D47 thigh7, D48 thigh6, D49 thigh5 

• You can download a simple servo alignment program here . This program sets your servos 
to center position. With your leg servos plugged in to digital pins 26 to 49 your robot should 
be standing with all hips perpendicular to the body. Thighs should be parallel to the ground 
and all legs perpendicular. Adjust your servo horns as best as you can. The servo 
positions can be fine-tuned in the software. 

• Because all the legs are made by hand, each leg has slightly different dimensions. Each 
servo center position will need to be adjusted slightly to compensate. 



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"Chopsticks" the Spider Robot 



Step 14 — Protecting your servos in the software. 




A 



• It is possible to damage a 
servo by trying to driving it 
past its physical limit. This photo 
shows the motor from a new servo. 
The brushed are burned and 
deformed because the motor 
regularly stalled from being over- 
driven. 

• The servo library for the Arduino 
allows you to set minimum and 
maximum limits for your servos 
when you use the servo.attach() 
function. This is an important 
feature as it can prevent a bug in 
your code from destroying your 
servos. 

• A positive pulse 1500uS wide sent 
every 20mS should center your 
servos. Different brands will accept 
different ranges of signal. 1000uS - 
2000uS is the standard but many 
servos can work beyond this 
range. 

• You should test your servos to 
determine the range of your input 
signal before the servo hits its 
limits. Use these values in the 
servo. attach() function to protect 
your servos. 



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"Chopsticks" the Spider Robot 



Step 15 — Using the sample code. 




• From here on it depends on how many legs you made and what sensors you use as to how 
your robot will turn out. 

• I remounted my diodes onto the shield to neaten the appearance. Then I added two pan/tilt 
brackets from DAGU along with four IR compound eyes. Each pair of eyes is wired in 
parallel to work as a single eye. As each eye can independently track an object at close 
range the robot gets a bit confused about which direction to go in sometimes. Video of it 
playing can be seen here . 

• To use my sample code you will have to tweak the servo center positions for the 3 basic 
positions, Sit, Stand and Curl Up. Make return; the first command of the ioop( ) 
function so that the robot holds its position after the servos initialize. 

• The variable pos must be initialized to the position you want the robot to stay in. 0=Sit, 
28=Stand and 56=Curl Up. 

• You can now adjust the servo center positions at the beginning of the program until the 
robot's position is correct. When you are happy with the positions, remove the return; 
command from the ioop( ) function and change pos back to 28. 

• You will also need to check which way your servos rotate. I changed knee servos recently 
and found they rotated in the opposite direction to the old servos. Fortunately the walk 
sequence code is easily modified. 

• If you are having problems with your robot and need help or want the latest code then go 
here . As well as myself, there are many people on LMR who are happy to help. 



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"Chopsticks" the Spider Robot 



Step 16 — Additional sensors. 



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"Chopsticks" the Spider Robot 



Simple Spicier sensors 



Gnd +5V Input 



100R 



IR 
LED., 



Gnd +5V Input 



180R 



IR 

photo 
transistor 



^$) Ar-i 



1/ 



LED 





o 



LDR 



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• The Spider controller has a lot of 
I/O pins. Don't be afraid to 
experiment. 

• Adding an IR LED and 
phototransistor to each foot will let 
the robot determine if it has come 
to an edge such as the edge of a 
table or the top of a set of stairs. 

• The LED shines infrared light onto 
the floor beneath the foot. Some of 
that light will reflect back and be 
detected by the phototransitor. If 
the foot goes over an edge then no 
light will be reflected back and the 
robot will know it cannot place a 
foot there. 

• If you want you can use normal 
LEDs and an LDR (light dependent 
resistor). In both cases the sensor 
will plug directly into the 3-pin 
servo connector. For these 
sensors to work the 20K internal 
pullup resistor must be enabled. 
When light hits the sensor it will 
pull the pin low. To enable the 
internal pullup resistor you must 
set that pin to high while the pin is 
configured as an input. 

• These sensors should work with 
both analog and digital inputs. 
Using the analog inputs will allow 
you to adjust the sensitivity in the 
software. 

• If you use an analog input and 
connect the LED to a digital output 

Page 20 of 21 



"Chopsticks" the Spider Robot 



instead of +5V then you can 
eliminate false readings due to 
ambient light. 

• Read your sensor with the LED on. 
This reading is ambient light + 
reflected light from the floor. Now 
read it again with the LED off. This 
is ambient light only. By 
subtracting the second reading 
from the first you will get a value 
that represents only light reflected 
from nearby objects such as the 
floor. Make sure your code allows 
time for the sensor to adjust when 
turning the LED on or off. 1mS 
should be plenty for 
phototransistors, LDRs will need at 
least 50mS. 



This project shows a relatively easy and low-cost approach to building robots. The combination 
of Polymorph and chopsticks will allow fairly complex robot chassis to be constructed that are 
strong, lightweight and low in cost. 

This document was last generated on 2012-11-02 12:49:51 AM. 



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